class RuboCop::Cop::Naming::ClassAndModuleCamelCase

end
module MyModule
end
class MyClass
# good
end
module My_Module
end
class My_Class
# bad
@example
an underscore in them.
This cops checks for class and module names with

def on_class(node) 

def on_class(node)
  return unless node.loc.name.source =~ /_/
  add_offense(node, location: :name)
end